(1) Field of the Invention
The present invention relates to the exchange of thermal energy between flowing fluids and particularly to the cooling of process gases such as those used during the thermic cracking of gaseous and liquid hydrocarbons. More specifically, this invention is directed to heat exchangers and especially to devices of such character which define generally coaxial isolated flow paths for a pair of fluids. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the prior art
Heat exchangers of the "double pipe" type, i.e., thermal energy transfer devices comprising at least one inner pipe and one outer pipe, are known in the art. In such heat exchangers, an example of which is enclosed in published German patent application No. 24 12 421, the medium to be cooled will typically flow through the inner pipe while the outer pipe, which is coaxial therewith and thus defines an annular flow path, has a cooling water-steam mixture passed therethrough. One or both pipes of the heat exchanger may be connected, at first ends, to a collector which has inner and outer chambers.
Heat exchangers of the type generally described above are used for the cooling of process gases, especially gases resulting from the thermic cracking of gaseous and liquid hydrocarbons. The cooling of such process gases is an important step in the production of ethyls and propyls. The stabilizing of the products of separation after the splitting process, which is directly related to the yield from the process, can only be achieved through the quick cooling of the gases. The requisite cooling requires that the velocity of the gases passing through the heat exchanger be relatively high and, of course, that there be very good transfer of heat, over the heating surface of the exchanger, from the process gas to the coolant.
An impediment to maximizing the efficiency of a thermic cracking process resides in the fact that particles of coke are formed during cooling of the cracked gases as a result of condensation of the simmering fractions of the hydrocarbon mixture. These coke particles are deposited on the walls of the heat exchanger partly because of the turbulence in the high velocity flow and also because of the existence of large temperature gradients across the heat exchanger cross-section. Such deposits reduce the heat exchange efficiency. The resultant rise in the process gas exit temperature has a negative effect on the entire production operation.
It has been common practice in the art to design "double pipe" heat exchangers, and particularly to size such devices, to achieve acceptable process gas exit temperature with moderate contamination of the walls of the pipe through which the process gas is flowing. Once the process gas exit temperature exceeds the acceptable level as a result of dirt accumulation on the pipe wall, the heat exchanger must be cleaned. The removal of the heat exchanger from service for cleaning dictates a loss of production and, consequently, is a very costly procedure.